1. Field of the Invention
The present invention relates to a home-use VTR (video tape recorder) for reproducing a video signal, and more particularly to a practical, integrated equalizer for a VTR, in which a change of equalizer characteristics does not affect a reproduction level of a video signal.
2. Description of the Prior Art
FIG. 1 shows an apparatus for reproducing a video signal which has been recorded on magnetic tape. A video signal is read from the magnetic tape 30 by a reproduction head 1 and supplied to a head amplifier 2 to be amplified before being applied to an equalizer 3. The equalizer 3 adjusts the frequency characteristics of the video signal so that the video signal's frequency characteristics are equal to those at the time of recording. After the video signal is equalized, it is applied to a subsequent FM demodulator 4 where a luminance signal is demodulated.
A typical equalizer is not usually an integrated circuit, and is generally comprised of discrete elements as shown in FIG. 2. Generally, a video signal is recorded with an overall frequency characteristic in which low and middle bands are enhanced, as is depicted in FIG. 3. In order to reproduce a video signal having such characteristics, the equalizer 3 must have the frequency characteristic shown in FIG. 4. In other words, the equalizer 3 reproduces a supplied video signal, based on the frequency characteristic shown in FIG. 4.
Referring to FIGS. 2, 4, and 5, a first trap circuit 5 removes a chroma signal (629 KHz, f1 in FIG. 4) from the video signal. A second trap circuit 6 removes any signals which have a frequency other than that of a video signal, that is, any out-band signals (f2 in FIG. 4). In this way, when the video signal passes through the first and second trap circuits 5 and 6, chroma and out-band signal components are removed. The remaining part of the video signal, that is, the luminance signal component, is then applied to the base of a transistor 7 which has its collector connected via a variable resistor 10 to a first band pass filter (BPF) 8, and its emitter connected to a second BPF 9. The first BPF 8 has a passband with center frequency f6 in FIG. 5 while the second BPF 9 has center frequency f7.
If the value of resistor 50 in FIG. 2 is adjusted, the characteristic curve of the first BPF 8 can be changed to the curve indicated by the chain line in FIG. 5. In this way, the overall characteristic curve shown in FIG. 5 can be transformed to match the curve in FIG. 4. When the characteristic curve of the first BPF 8 is changed, the level of a signal output from the first BPF 8 is also changed. This change in signal level, however, can be avoided by adjusting the variable resistor 10, which can be done easily, since the circuit in FIG. 2 has a discrete structure. More specifically, if the carrier component of the luminance signal has level A in FIG. 5, the level A can remain unchanged despite a change in the characteristic curve of the first BPF 8.
Since the characteristic curve shown in FIG. 5 varies depending on the design of a VTR, the variable resistor 10 is necessary for adjustment to match different input signals. As discussed above, because the circuit in FIG. 2 has a discrete structure, the characteristics thereof can be easily changed, however additional cost and layout space are used.
In order to solve this problem, the circuit in FIG. 2 has been rearranged as is shown in FIG. 6 to allow for circuit integration. The new circuit, that is, integrated circuit, comprises a first BPF 18 and a second BPF 19. The first BPF 18 has a characteristic curve indicated by solid line BPF 1 in FIG. 7, center frequency set at f6, while the second BPF 19 has a characteristic curve indicated by solid line BPF 2, center frequency set at f7. Referring again to FIG. 6, the circuit further comprises an adder circuit 11 for adding outputs from the first and second BPFs 18 and 19, the output of the adder circuit 11 is then sent through first and second trap circuits 5 and 6 to remove extra signals as described above.
As described above, in order to match the characteristic curve shown in FIG. 7 to the curve shown in FIG. 4, a variable resistor 12 is provided as shown in FIG. 6. When the value of the variable resistor 12 is adjusted, the characteristic curve BPF 1 indicated by a solid line in FIG. 7 can be modified so as to form the curves indicated by dotted lines in the same figure. In the circuit structure of FIG. 6, if the variable resistor 12 is provided outside of the integrated circuit, the frequency characteristic can be easily adjusted, and this circuit has a function equivalent to that of the circuit in FIG. 2.
The circuit of FIG. 6, however, has a problem in that the level A of the carrier component of the luminance signal is raised to level A2 by the adjustment of variable resistor 12 as shown in FIG. 7. If the level of the carrier component shifts from level A to level A2, the luminance signal has a larger dynamic range before it is applied to the FM demodulator 4. As a result, a discrepancy results between the characteristics of the signal and a conventional FM demodulator 4, which will consequently require the later to be re-designed.
Because of the above problems, an equalizer which can be easily integrated, and in which the signal level remains unaffected by a change of frequency characteristic is proposed.